Fe3O4@nano-almondshell/Si(CH2)3/2-(1-piperazinyl)ethylamine as an effective magnetite almond shell-based nanocatalyst for the synthesis of dihydropyrano[3,2-c]chromene and tetrahydrobenzo[b]pyran derivatives

The preparation and design of nano-catalysts based on magnetic biopolymers as green and biocompatible nano-catalysts have made many advances. This paper deals with the preparation of magnetite biopolymer-based Brønsted base nano-catalyst from a nano-almond (Prunus dulcis) shell. This magnetite biopolymer-based nano-catalyst was obtained through a simple process based on the core-shelling of nano-almond shell and Fe3O4 NPs and then the immobilization of 3-chloropropyltrimethoxysilane as linker and 2-aminoethylpiperazine as a basic section. Structural and morphological analysis of this magnetite biopolymer-based nano-catalyst were done using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, X-ray diffraction, Thermogravimetric analysis, Vibrating sample magnetization, Energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller, and Transmission electron microscopy techniques. The performance of the synthesized Fe3O4@nano-almondshell/Si(CH2)3/2-(1-piperazinyl)ethylamine as a novel magnetite biopolymer-based nano-catalyst for the synthesis of dihydropyrano[3,2-c]chromene and tetrahydrobenzo[b]pyran was investigated and showed excellent efficiency.


Results and discussions
In this paper, the FNASiPPEA, a magnetite biopolymer-based nano-catalyst, was used as an environmentally friendly basic nano-catalyst for the synthesis of dihydropyrano [3,2-c]chromene (DHPC) and tetrahydrobenzo [b] pyran (THBP) derivatives through a multicomponent reaction under optimized conditions. The FNASiPPEA was first prepared by preparing Fe 3 O 4 @nanoalmondshell according to previously reported methods 42 . After www.nature.com/scientificreports/ that, FNASiPPEA was prepared by immobilization of 3-chloropropyltrimethoxysilane (CPTMS) and finally 2-aminoethylpiperazine (AEP) (as a base agent) on the surface of the nano-catalyst.
FT-IR of FNASiPPEA. FT-IR spectra of Fe 3 O 4 @nanoalmondshell, AEP, and FNASiPPEA are shown in Fig. 2. FT-IR spectrum of nano-almondshell (Fig. 2a) shows distinct peaks at 3428 cm -1 , 2920 cm -1 , and 1122 cm -1 , which are related to O-H, C-H, and C-O vibrational stretching, respectively. In the FNASiPPEA spectrum (Fig. 2c), a distinct peak at 588 cm -1 is attributed to the Fe-O stretching vibration. Also, the broad peak at the range of 3400 cm -1 is attributed to the stretching vibration of N-H, which overlaps with the stretching vibration of the O-H group. CPTMS immobilization on Fe 3 O 4 @nanoalmondshell is confirmed by a characteristic peak at 1111 cm -1 , which corresponds to the Si-O stretching vibration. The characteristic peak at 1451 cm -1 is related to C-N stretching vibration.
FESEM and TEM of FNASiPPEA. The surface morphology and detailed structure of the FNASiPPEA nano-catalyst were investigated using FESEM (Fig. 3). Figures 3a and b show the average particle size of the catalyst (11-43 nm) which appeared as nanospheres with pseudo-spherical morphology. The intrinsic structure was characterized using TEM measurements (Fig. 3c) which show core-shell nanoparticles.
PXRD (Powder X-ray diffraction) of FNASiPPEA. Figure 4 shows the XRD patterns of Fe 3 O 4 NPs and magnetite biopolymer-based FNASiPPEA nanoparticles. All the diffraction peaks appearing at 2θ = 31°, 35°, 43°, 54°, 57°, and 63° in the spectrum (4a) can be indexed as centered cubic Fe 3 O 4 , which agrees well with the reported data correspond 43 . In the XRD pattern (4b), a new peak appears at 2θ = 23° and a broad peak at 2θ = 20-30°, which is due to the presence of nano-almondshell and amorphous silica, respectively.
TGA of FNASiPPEA. Figure 5 shows the TGA and DTA curves of magnetite FNASiPPEA. The nano-catalyst shows a small initial mass reduction at a temperature lower than 100 °C due to the removal of absorbed water and other organic solvents. At temperatures higher than 100 °C, (180-370 °C) the highest weight loss is observed in the TGA curve, which was probably due to the decomposition of nano-almondshell and organic parts (amine groups and methoxy groups) from the catalyst.
VSM of FNASiPPEA. The magnetic properties of the FNASiPPEA were evaluated in Fig. 6. The magnetic curve shows no remanence and coercivity, indicating the nanocatalyst's superparamagnetic behavior. The saturation magnetization value of FNASiPPEA (33 emu/g) is lower than that of Fe 3 O 4 (47 emu/g). The low magnetization of the catalyst is attributed to the non-magnetic functionalized nano-almond shell coating on Fe 3 O 4 NPs. However, the magnetic susceptibility of FNASiPPEA is strong enough to be separable by an external magnet from the reaction medium.     obtained by nitrogen adsorption and desorption measurements (Fig. 9). The N 2 isotherms related to the type IV isotherm in the IUPAC classification have shown H 3 type rings, which can indicate the existence of mesopores and also have non-hard pores. As shown in Table 1 a s , BJH (Barrett-Joyner-Halenda), and pore diameter were 7.0116 m 2 g -1 , 0.050029 cm 3 g -1 , and 28.206 nm respectively. All the above results confirm the successful synthesis of magnetite biopolymer-based FNASiPPEA. After the detailed description of the prepared nano-catalyst, its catalytic performance was investigated for DHPC synthesis. Therefore, different reaction conditions such as the amount of catalyst, solvent, and temperature were investigated for a model reaction between 4-nitrobenzaldehyde, 4-hydroxycoumarin, and malononitrile ( Table 2). While screening the model reaction in different solvents such as H 2 O, EtOH, and H 2 O/EtOH, the best result was obtained in the EtOH solvent (Table 2, entry 10).
After optimizing the reaction conditions, to determine the application range of FNASiPPEA, various aldehydes were used in the reaction. The results are summarized in Table 3.
Then, optimization of the reaction conditions for the synthesis of THBP was carried out. Hence, the reaction between 4-nitrobenzaldehyde, dimedone, and malononitrile in the presence of 0.02 g of catalyst at 50 °C in solvent-free conditions has been adopted ( Table 4, entry 9). The results are summarized in Table 4.
We used various aldehydes in the reaction to investigate the application range of FNASiPPEA as a magnetite biopolymer-based nano-catalyst. The result is presented in Table 5.
To compare the efficiency of this magnetite biopolymer-based nano-catalyst with other catalysts for the synthesis of DHPC and THBP derivatives, a summary of the results was collected in Tables 6 and 7. As can be seen in Tables 6 and 7, the reaction efficiency of this catalyst is better than other catalysts and the reaction time is shorter than others.

Hot filtration test.
Because the FNASiPPEA is a heterogeneous nano-catalyst, a heterogeneity test called hot filtration was performed. In this way, first, the reaction was allowed to continue in the presence of the FNASiPPEA nano-catalyst, and then after half the time, the catalyst was removed from the reaction mixture and continued the reaction, as can be seen in Fig. 10a, no reaction progress was observed in the absence of the nano-catalyst, which indicates no leakage of solid catalyst into the reaction mixture. Therefore, the FNASiPPEA nano-catalyst is heterogeneous and suitable for DHPC and THBP synthesis reactions without any leaching.

Reusability of FNASiPPEA.
To check the recyclability of the catalyst, after the completion of the reaction, the catalyst can be separated from the reaction mixture with a magnet, and after washing with chloroform (CHCl 3 ) and drying at the ambient temperature, and then can be reused for the synthesis of DHPC and THBP, which includes aldehyde (1 mmol), 1,3-diketone (4-hydroxycoumarin, dimedone), (1 mmol), and malononitrile (1.5 mmol) under optimized conditions. Therefore, the reusability of the catalyst for the model reaction was evaluated for the synthesis of DHPC and THBP (Figs. 11 and 12). FT-IR, XRD, VSM, and FESEM analyses of the nano-catalyst recovered after the 3rd run was also performed. According to Figs. 10b, c, d, and e, the matching

Proposed mechanism for synthesis of dihydropyrano[3,2-c]chromene and tetrahydrobenzo[b]
pyran. Figure 13 shows the possible mechanism for the synthesis of dihydropyrano[3,2-c]chromene and tetrahydrobenzo[b]pyran derivatives using FNASiPPEA as a magnetite Brønsted base nano-catalyst. First, the Knoevenagel condensation between malononitrile and aldehyde is followed by loss of water to form an intermediate (a). Then the Michael addition between the intermediates (a) and (b) (dimedone, 4-hydroxycoumarin) and then intramolecular cyclization and tautomerization in the presence of the catalyst leads to the production of the corresponding product.

Experimental section
Materials and methods. Chemicals were purchased from Merck, Fluka, and Aldrich Chemical Companies. 1 H NMR and 13 C NMR spectra were recorded at 400 and 100 MHz, respectively. Fourier transform infrared (FT-IR) measurements (in KBr pellets or ATR) were recorded on a Brucker spectrometer. Melting points were determined on a Büchi B-540 apparatus. The X-ray diffraction (XRD) pattern was obtained by a Philips Xpert MPD diffractometer equipped with a Cu Kα anode (k = 1.54 Å) in the 2θ range from 10° to 80°. Field Emission Scanning Electron Microscopy (FESEM) was obtained on a Mira 3-XMU. VSM measurements were performed by using a vibrating sample magnetometer (Meghnatis Daghigh Kavir Co. Kashan Kavir, Iran). Energy-dispersive X-ray spectroscopy (EDS) of nano-catalyst was measured by an EDS instrument and Phenom pro X. The EDX-MAP micrographs were obtained on the MIRA II detector SAMX (France). Thermal gravimetric analysis (TGA) was conducted using the "STA 504" instrument. Transmission electron microscopy (TEM) was obtained using a Philips CM120 with a LaB6 cathode and an accelerating voltage of 120 kV. BELSORP MINI II nitrogen adsorption apparatus (Japan) for recording Brunauer-Emmett-Teller (BET) of nano-catalyst at 77 K.

Preparation of nano-almondshell.
To prepare the nano-almondshell, the almondshell was heated in boiling water for 30 min, dried, and powdered. After that, was treated with a 17.5 w/v NaOH solution at 90 °C for 24 h under reflux conditions. Subsequently, the almondshell was filtered and washed with distilled water until the alkali was eliminated. Then, bleached with 100 mL of 1:1 aqueous dilution of 3.5% w/v sodium hypochlorite (NaOCl) at 80 °C for 3 h under reflux conditions. The resulting almondshell particles were hydrolyzed partially using the 35% sulfuric acid (H 2 SO 4 ) aqueous solution with an almondshell-to-acid weight ratio of 1-10 at 45 °C. After 3 h, the obtained suspension was diluted with water five-fold to stop the hydrolysis reaction. The suspension was centrifuged at 4000 rpm to separate the nano-almondshell from the acidic medium (yield 60%).

Synthesis of Fe 3 O 4 @nano-almondshell/Si(CH 2 ) 3 Cl (FNASiP-Cl).
In a 100 mL flask, 1 g of dried  Table 2. After the end of the reaction (TLC, n-hexane: ethyl acetate 6:4), the catalyst FNA-SiPPEA was separated from the reaction mixture by an external magnet, the solvent was removed under reduced pressure, and the precipitate was washed with methanol and recrystallized with chloroform for further purification.    (Table 4). After the completion of the reaction (TLC, n-hexane: ethyl acetate 7:3), the magnetite catalyst was removed from the reaction mixture by a magnet, the solvent was removed under reduced pressure, and the product was obtained after washing and recrystallization with chloroform.

Conclusion
In summary, the magnetite almondshell-based nano-catalyst was prepared, characterized, and used for the synthesis of DHPC and THBP. The prepared nano-catalyst FNASiPPEA shows high catalytic activity and good reusability. Meanwhile, this method is non-toxic and biodegradable, it may be used to prepare other biopolymerbased nano-catalysts for more interesting reactions.

Data availability
The datasets generated and/or analysed during the current study are available in this article and its supplementary information files.